Wafer-level underfill process making use of sacrificial contact pad protective material

ABSTRACT

A method for connecting electronic components, such as, an integrated circuit die and a package substrate, is described. According to one aspect of the invention, a contact pad protective material is applied on one or more of the contact pads on an integrated circuit die. The underfill material is applied to the surface of the die not covered by the contact pad protective material and the underfill material is partially cured in a curing oven. The contact pad material is removed leaving openings over the respective surface of the contact pad. A one or more contacts on a package substrate is inserted into the openings, electronically connecting the contacts to the contact pads.

CROSS-REFERENCE TO OTHER APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 10/165,551,filed on Jun. 7, 2002, issued as U.S. Pat. No. 7,059,048 on Jun. 13,2006.

BACKGROUND

1. Field

Embodiments of the invention relate to the field of integrated circuitpackaging and, more specifically, to microprocessor flip chip packaging.

2. Background

One method of connecting a semiconductor die onto a package substrate isknown in the art as controlled collapse chip connection (C4) flip chipmanufacturing. Flip chip manufacturing includes a semiconductor diehaving contact pads (e.g., electric terminals) placed on one side of thedie so that it can be flipped over and bonded with adhesive with thecontact pads providing an electrical connection with contact bumps on apackage substrate. Such adhesives are typically filled fine silicaparticles in order to compensate for differences in coefficient ofthermal expansion (CTE) between flip chip components. It is thereforedesirable that adhesive formulations have high filler concentrations soas to more closely match the CTE of the components comprising the flipchip device.

A common method of applying such adhesives is to dispense a small amountof liquid adhesive along the edge of the die allowing it to flow underthe die via capillary action. While this process has proven to be usefulit has several limitations related to cost and adhesive formulation.Cost is relatively high because the assemblies must be individuallyhandled numerous times in order to dispense the adhesive. The amount offiller that can be formulated into such as adhesives is quite limitedbecause the material must have a relatively low viscosity in order toeffectively flow under the die via capillary action. Typical capillaryunderfilling adhesive materials are limited to filler concentrations nogreater than 70% by weight, thus limiting the ability to tailor the CTEof the material to values more closely matching those possessed by theother components comprising the flip chip device.

In addition, problems occur when the adhesive used to bond the die withpackage substrate sets between the contact pads and contact bumps. Forexample, when the adhesive is set between the contact pads and thecontact bumps, the electrical connection between the die and the packagesubstrate is lessened. That is, the adhesive interferes with theelectrical connection between the die and the package substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 illustrates a process flow of an underfill coating process usinga contact pad protective material according to one embodiment of theinvention;

FIG. 2A is a cross-section view of a contact pad protective materialstencil printing detail according to one embodiment of the invention;

FIG. 2B is a cross-section view of contact pad protective materialstructures printed onto a wafer contact pad surface according to oneembodiment of the invention;

FIG. 2C is a cross-section view of an underfill stencil printing detailaccording to one embodiment of the invention;

FIG. 2D is a cross-section view of partially cured underfill coatingsurrounding the contact pad protective material in a cure oven accordingto one embodiment of the invention;

FIG. 2E is a cross-section view of a solvent extraction system used toremove the contact pad protective material according to one embodimentof the invention;

FIG. 2F is a cross-section view of a diced wafer yielding die withpartially cured underfill coating and exposed contact pad areasaccording to one embodiment; and

FIG. 2G is a cross-section view of an integrated circuit assemblyaccording to one embodiment.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

A method for applying an underfill material between electroniccomponents, such as an integrated circuit die and a package substrateusing a contact pad protective material, is described. FIG. 1 is usedwith FIGS. 2A, 2B, 2C, 2D, 2E, 2F, and 2G to illustrate one embodimentof an underfill coating process using a contact pad protective material.

At 110, a contact pad protective material 50 is applied on one or morecontact pads 20 on a wafer 22 in FIG. 2A, the wafer 22 having aplurality of silicon die and, each die having one or more contact padsas described. The contact pad protective material 50 covering thecontact pads 20 is to prevent an underfill material from covering one ormore contact pads 20, as will be described. The contact pad protectivematerial 50 may be stencil printed onto the surface of wafer 22 with acontact pad protective material stencil 33. The contact pad protectivematerial stencil 33 may be manufactured so as to have contact padprotective material stencil apertures 32 of appropriate design forapplying the contact pad protective material 50 onto the correspondingcontact pads 20 on the wafer 22 using a stencil squeegee 31 to applycontact pad protective material 50. However, it should be understoodthat embodiments of the invention are not limited to using the contactpad protective material stencil 33 to applying the contact pad material50, and in alternative embodiments other techniques, such as screenprinting, which is well known to those of ordinary skill in the art,might also be used within the scope of the embodiments.

Contact pad protective material 50 is formulated so as to allow for theformation of contact pad protective material structures 52 on eachcontact pad 20, as shown in FIG. 2B. The contact pad protective materialstencil 33 may be fabricated in accordance with a desired contact padprotective structure 52, pitch, and height. For example, a contact padprotective structure may have a pitch range greater than 150 um and aheight in the range of 20 to 30 um or greater.

In one embodiment, the contact pad protective material 50 may comprise apolymer, such as polystyrene in a non-aqueous solution with a solventsuch as acetone, as well as an aqueous solution of polyvinyl alcohol,among other examples. Typically such materials have viscosity in therange of 1000 to 10000 cPS allowing for patterning and formation of thecontact pad protective material structures 52 using stencil printing orsimilar processes.

At 120, an underfill material 60 is applied over the unprotected surfaceof wafer 22 via underfill stencil 35, as shown in FIG. 2C. Underfillstencil 35 is manufactured so as to have underfill stencil apertures 37and a printing pattern appropriate for placing the underfill material 60on the wafer between the contact pad protective material structures 52using a underfill stencil squeegee 36. The underfill material 60 may becomprised of the general class of materials known as epoxies, includingan amine epoxy, an imidizole epoxy, an phenolic epoxy or an anhydrideepoxy system, among similar chemistries well known to those of ordinaryskill in the art. In addition, other examples of underfill material 60may include underfills such as benzocyclobutene (BCB), a bismalleimidetype underfill, a polybenzoxazine system, or a polynorborene typeunderfill, among other examples. Also, the underfill material 60 mayoptionally be filled with inorganic fillers such as silica to controlthe thermal expansion of material 60. It should be appreciated that theunderfill material 60 does not have to exhibit good capillary flow inthe embodiments of this invention. Therefore, if a filler is used, itcan be used at a much higher level fill than a normal capillary styleunderfill. This in turn allows for greater freedom to achieve low CTE inunderfill material 60 through higher filler loadings.

At 130, the underfill material 60 is partially cured to a substantiallydense form as shown in FIG. 2D. Here, the wafer 22 coated with underfillmaterial 60 is placed in a curing oven 70 (e.g., a thermal cure oven, amicrowave oven, etc.) in order to partially cure the underfill material60. In one embodiment, the time, temperature, and environmentalconditions of curing the oven 70 depend on the contact pad protectivematerial 50, underfill material chemistry, and the degree of curerequired.

For example, if underfill resin 60 is an epoxy material and the targetdegree of cure is in the range of sixty to eighty percent, the curingoven 70 conditions are in the following range: (1) time=ten to sixtyminutes; (2) temperature=forty to sixty degrees Celsius; and (3)environmental conditions=ambient air. Maximum curing temperature shouldbe adjusted in accordance with the melting point of the contact padprotective material 50. The curing temperature for the underfillmaterial 60 should be less than the melting point of the contact padprotective material 50. For example, the underfill partial curingtemperature should be ten to twenty degrees Celsius below the contactpad protective material 50 melting point.

At 140, the contact pad protective material 50 is removed from thecontact pad 20, as shown in FIG. 2E. Here, the contact pad protectivematerial 50 is removed from the contact pads 20 with a solventextraction system 80 to expose the contact pads 20 through openings inthe underfill material. The wafer 22 is placed in the solvent extractionsystem 80 (e.g., acetone) to remove the contact pad protectivestructures 52. The time and temperature to remove the contact padprotective structures 52 in the solvent extraction system 60 may dependon the solvent, the contact pad protective material 50, and theunderfill material 60 chemistry and degree of cure. For example, ifunderfill material 60 is an epoxy material and the target degree of cureis in the range of sixty to eighty percent, and the contact padprotective material 50 is a polystyrene, the solvent extraction systemconditions are in the following range: (1) time=ten to five hundredseconds; and (2) temperature=room temperature to forty degrees Celsius.Maximum extraction temperature may be adjusted in accordance with thecontact pad protective material 50 and the solvent agent.

At 150, the wafer 22 is separated or diced into individual die. Theprocess of yielding a diced wafer is well known to those of ordinaryskill in the art. FIG. 2F shows a diced wafer 22 having a cutaway viewof diced region 70. Region 70 shows three die 10 that have been cut fromthe wafer 22, each diced die 10 having exposed contact pads 20 betweenthe partial cured underfill material 60.

In one embodiment, the wafer street areas, signifying where each die isto be cut from the wafer 22, may be kept clear of the underfill material60 and the contact pad protective material 50, so as to avoid problemswith sawing caused by blade loading with debris. As stated above,contact pad protective material 50 may also be applied to the waferstreet areas so as to provide a means of protecting these areas fromdownstream wafer sawing operations. Therefore, as with contact padprotective material 50 placed over the contact pads 20, contact padprotective material 50 protecting street areas is also removed when thecontact pad protective material 50 is removed so as to exposedebris-free areas for sawing. Alternate embodiments of the presentinvention include performing wafer-sawing operations prior to applyingthe contact pad protective material 50 to the wafer as described inblock 110. In either example, wafer 22 is sawn along kerf area 70yielding a plurality of diced die 10 from wafer 22 to be attached to apackage substrate.

In one embodiment, the plurality of diced die 10 are made available fordownstream processing by leaving diced wafer 22 on a wafer ring andfilm. The resultant sub-assembly is placed in a conventional pick andplace machine from which each die 10 are picked from the wafer ring andplaced onto bumped substrate 40 having corresponding contact bumps 30.

At 160, the die 10 having the underfill material with openings to thecontact pads 20 are flip-chip attached with the package substrate 40having the contact bumps 30, as shown in FIG. 2G. Here, the contactbumps are inserted into the openings in the underfill material toprovide an electrical connection with the contact pad. For example, thediced die 10 might be picked from wafer 22 and flipped for assembly by aprocess well known to those of ordinary skill in the art. Alternately,the die can also be singluated and placed on a tape and reel typestorage and delivery system before being picked and flipped for finalassembly by a process well known to those of ordinary skill in the art.

In order to complete the electrical interconnection between die 10 andsubstrate 40, integrated circuit assembly 5 is placed in a reflow ovenwell known to those of ordinary skill in the art, in accordance withsubstrate bump metallurgy. For example, if the substrate bumps 30 arecomposed of eutectic solder (Pb/Sn 63/37), reflow conditions aretypically as follows: soak at 150-160 C.; ramp to peak temperatures of218-222 C. for ninety seconds; and total time is approximately fiveminutes. Reflow simultaneously causes substrate bumps 30 to flow ontothe die contact pad 20 and causes the underfill 60 to cure.Alternatively, a fluxing agent or solder paste cure may be placed in theopen contact pads to aid in the flip chip attachment process. Here, thetop surface of the contact pad 20 may be soldered with the top surfaceof the contact bump 30.

It should be understood that although one or more contact pads 20 areshown on the die 10, and one or more contact bumps 30 are shown on thesubstrate 40, in an alternative embodiment, the one or more contact pads20 are on the substrate 40, and one or more contact bumps 30 are on thedie 10. Therefore, either the package substrate or wafer may be treatedwith the contact pad protective material and pre-cured with theunderfill material as described, depending on the configuration of thewafer or the package substrate. In addition, although an embodiment ofthe invention has been described with contact bumps connecting to thecontact pad, in alternative embodiments of the invention, alternativeexamples of connection techniques and contacts might be used such ascontact pens among other examples well known to those of ordinary skillin the art. For example, according to yet another embodiment of theinvention, substrate bumps 30 may be composed of electrically conductivepolymeric materials, such that, the final connection between contactpads 20 and substrate bump 30 is achieved by adhesive bonding by usingappropriate adhesive resin, latency of the resin comprising the matrixof polymer bump 30, or a combination thereof.

According to one embodiment of the invention, the process includes theapplying of a photo-definable underfill material system onto the wafersurface having contact pads. The photo-defined underfill material maycomprise a benzocyclobutene (BCB). In this way, using an appropriatemask pattern, the film is photo-defined so the underfill material lyingbetween contact pads is partially cured. The resultant surface isexposed to a suitable solvent system so that the areas lying over thecontact pad surfaces are dissolved, exposing the contact pad surface forflip chip attachment.

For example, a suitable wafer underfil material may be Dow 4021-46, aphoto-definable BCB formulation that is printed on the wafer surface,and photo-defined and partially cured to seventy-five to eighty degreeof cure. Exposure of the resultant structure to a polar solvent, such asa fifty-fifty blend of water and ethanol, will result in the dissolutionof PVA contact pad protective material.

According to another embodiment, ablation technology, well known tothose of ordinary skill in the art, may be used. Here, an underfillmaterial is applied on the wafer including the contact pads. Theunderfill material areas lying over the contact pads' surfaces areablated by either energized light (e.g., carbon dioxide laser) or plasma(e.g., argon) so that the areas lying over contact pad surfaces areexposed for subsequent flip chip attachment.

It should also be understood that the embodiments described provide fora process to interconnect an integrated circuit assembly, such that theunderfill material is not between the contact pad and the contact bumps.In addition, these embodiments provide a means for applying a highlyfilled and CTE-tailorable material onto the wafer surfaces allowing forconventional flip chip manufacturing techniques to be used, andtherefore allows for high throughput and low cost manufacturing.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described. The method and apparatus of theinvention can be practiced with modification and alteration within thespirit and scope of the appended claims. The description is thus to beregarded as illustrative instead of limiting on the invention.

1. A method of constructing an electronic assembly, comprising: applying a contact pad protective material on a contact pad, the contact pad being on a surface of a first electronic component; applying an underfill material, different from the contact pad protective material, over the surface of the first electronic component; removing a portion of the contact pad protective material from the contact pad to form an opening that exposes the contact pad on the first electronic component; and connecting the first electronic component to a second electronic component having a contact, the connecting includes inserting the contact into the opening that exposes the contact pad to provide an electrical connection between the contact and the contact pad, wherein applying the underfill material further comprises using a underfill stencil to stencil print the underfill material onto the surface of the first electronic component.
 2. The method of claim 1 wherein applying the contact pad protective material further comprises using a contact pad protective material stencil to stencil print the contact pad protective material onto the contact pad.
 3. The method of claim 1 wherein the contact pad protective material is polymer.
 4. The method of claim 3 wherein the contact pad protective material is a acetone solution of polystyrene.
 5. The method of claim 1 wherein the contact pad protective material is an aqueous solution of polyvinyl alcohol.
 6. The method of claim 1 wherein the underfill material is an epoxy.
 7. The method of claim 1 wherein the underfill material is an amine epoxy.
 8. The method of claim 1 wherein the underfill material is a benzocyclobutene epoxy.
 9. The method of claim 1 wherein the curing temperature for the underfill material is less than the melting point of the contact pad protective material.
 10. The method of claim 1 wherein the removing the contact pad protective material further comprises exposing the contact pad protective material to a solvent system.
 11. The method of claim 10 wherein the solvent system includes acetone.
 12. The method of claim 1 wherein the connecting further comprises performing flip chip processing.
 13. The method of claim 1 wherein the connecting further comprises: re-curing the underfill material to connect the surface of the first electronic component with the second electronic component.
 14. The method of claim 1 wherein the connecting further comprises: soldering together the contact pad and the contact using a soldering material.
 15. The method of claim 1 wherein the first electronic component is one of a plurality of integrated circuit die on a wafer, wherein the second electronic component is a package substrate, wherein the connecting the first electronic component further comprises dicing the first electronic component from the wafer prior to connecting the first electronic component to the second electronic component. 